There are several views on this topic and some will be presented here based on analysis of the Holy Bible.

The most explicit description is in the Book of Revelation.

“John was shown a vision of a “sign…in heaven…a great fiery red dragon” (Rev.12:3), in which the Archangel Michael and his angels wage battle against this serpent. “So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him.”(v.9).

The same passage speaks of the dragon sweeping “a third of the stars of heaven” down with his tail (Rev. 12:4) Many commentators interpret the stars as angels – indicating that Satan caused a third of the heavenly hosts to rebel with him when he arrogantly tried to be like god. Since the angels are without number, “ten thousand times ten thousand, and thousands of thousands” (Rev. 5:11) the sheer number of Satan’s angels is beyond our mind’s grasp.

These angels are demons – or unclean spirits – the Bible speaks of. Their name comes from the Latin word daemon, meaning ”evil spirit”, and the from Greek word daimon, meaning “a divinity”.
The above is quoted directly from the Legacy of Faith Study Bible with teachings from Dr. Lester Sumrall. www.lesea.org

Another view is based on a Pre-Adamic race that was wiped out by a flood.

“Most theologians who are spirit filled would suggest that demons are the Pre-Adamic race, in that in Genesis 1, the Holy Trinity hovered over the face of the waters, indicating that the earth was flooded. Upon reading the account of Noah in Genesis, the Lord enacts a covenant to "never again" flood the earth, instead bringing about Jesus the Christ through the now, new, lineage through Noah's 3 sons.”
Pastor Tim Waugh

Amp Gen 1:1 IN THE beginning God (prepared, formed, fashioned, and) created the heavens and the earth. [Heb. 11:3.]
Amp Gen 1:2 The earth was without form and an empty waste, and darkness was upon the face of the very great deep. The Spirit of God was moving (hovering, brooding) over the face of the waters.

Hebrew Mechanical Translation
Genesis 1:1 - 2 in the summit “Elohiym [Powers]” fattened the sky and the land, and the land had existed in confusion and was unfilled and darkness was upon the face of the deep sea and the wind of “Elohiym [Powers]” was much fluttering upon the face of the water
Excerpt from http://www.ancient-hebrew.org/

Amp Gen 9:11 I will establish My covenant or pledge with you: Never again shall all flesh be cut off by the waters of a flood; neither shall there ever again be a flood to destroy the earth and make it corrupt.
Amp Gen 9:12 And God said, This is the token of the covenant (solemn pledge) which I am making between Me and you and every living creature that is with you, for all future generations:
Amp Gen 9:13 I set My bow [rainbow] in the cloud, and it shall be a token or sign of a covenant or solemn pledge between Me and the earth.

Another view Proposes that angels and human women had children.

Amp Gen 6:1 WHEN MEN began to multiply on the face of the land and daughters were born to them,
Amp Gen 6:2 The sons of God saw that the daughters of men were fair, and they took wives of all they desired and chose.
Amp Gen 6:3 Then the Lord said, My Spirit shall not forever dwell and strive with man, for he also is flesh; but his days shall yet be 120 years.
Amp Gen 6:4 There were giants on the earth in those days--and also afterward--when the sons of God lived with the daughters of men, and they bore children to them. These were the mighty men who were of old, men of renown.

Genesis 6: 1-4 Analysis
“Some people have thought that the “sons of God” were fallen angels. But the “sons of God” were probably not angels, because angels do not marry or reproduce (Mathew 22:30; Mark 12:25). Some scholars believe this phrase refers to the decendants of Seth who intermarried with Cain’s Evil Descendants. This would have weakened the the good influence of the faithful and increased the moral depravity in the world, resulting in an explosion of evil.”
Excerpt from the Life Application study Bible, New Living Translation second edition, Tyndale House Publishers, Inc. Wheaton, Illinois.

What do demons want from people?

"Demons are personalities without bodies, and they are highly organized. As fallen spirits, they desire to dwell in a body in order to manifest themselves. They are angry with God because of their fallen state, and their prime motive is to destroy what God loves or creates - chiefly man.

Demons often state the names of people they have lived in previously. When someone who is possessed of a devil dies, that spirit immediately seeks a dwelling place in another person. However, he cannot walk into just any person's life; he must find an open door. If he is a spirit of anger, he seeks to possess a person who has little control over his temper. A spirit of insanity will seek to enter a person's troubled mind."
The above is quoted directly from the Legacy of Faith Study Bible with teachings from Dr. Lester Sumrall. www.lesea.org

Can an evil spirit enter into and posses a Christian?

"The Bible emphaticaly states in James 4:7, "Resist the devil and he will flee from you." This is the position of a born-again believer. Jesus, in His ultimate commission to the church said, "'In My name they will cast out demons'" (Mark 16:17). The Christian should not fear the devil; he should cast him out, dethrone him, and exorcise him.
However, those who identify themselves as Christians and go to fortune-tellers, play the Ouija board and depend of the horoscopes for guidance are opening a door for the entrance of the evil one."
The above is quoted directly from the Legacy of Faith Study Bible with teachings from Dr. Lester Sumrall. www.lesea.org

What is the Future of the devil and his demons?

NKJV Revelation 20:10
The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever.